1. Technical Field
This invention relates to human cytomegalovirus (HCMV), and in particular to peptide fragments from a single subunit protein that function as T-cell epitopes of HCMV in human beings. The peptide fragments are capable of directing human cytotoxic T lymphocytes (CTL) to recognize and lyse human cells infected with HCMV. The peptide fragments can independently direct HCMV-specific CTL to lyse cells incubated with the peptide and which express HLA A, B or C genes.
2. Description of the Background Art
The HCMV genome is relatively large (about 235 k base pairs) and has the capacity to encode more than two hundred proteins. HCMV is composed of a nuclear complex of nucleic acid (double-stranded DNA) surrounded by capsid proteins having structural or enzymatic functions, and an external glycopeptide- and glycolipid-containing membrane envelope. HCMV is a member of the herpes virus family and has been associated with a number of clinical syndromes.
HCMV infection is relatively common and is usually self-limiting in the healthy, immunocompetent child or adult (L. Rasmussen, Curr. Top. Microbiol. Immunol. 154:221-254, 1990). Approximately ten percent (10%) of all newborn infants carry HCMV and the virus can cause severe congenital disease in the fetus or infant. Some of these newborn infants suffer congenital birth defects. Other newborn infants carry cytomegalovirus for some time before they actually show symptoms of the disease. For example, HCMV is a common cause of mental retardation in children who acquire the infection in utero from mothers carrying an active infection.
Several studies have begun to question whether persistent and apparently asymptomatic HCMV infection in an otherwise healthy adult poses health risks in certain individuals. For example, individuals who have undergone coronary angioplasty sometimes subsequently develop restenosis as a result of arterial remodeling. In one study, about one third of such patients with restenosis had detectable HCMV DNA in their arterial lesions (E. Speir et al., Science 265:391-394 (1994)), whereas in another study CMV seropositive patients were five times more likely to develop restenosis than their seronegative counterparts (Y. F. Zhou et al., New England J. Med. 335:624-630 (1996)). These studies suggest that decreasing the number of HCMV infected host cells can benefit certain individuals.
HCMV also has been associated with morbidity and mortality in immuno-compromised patients. HCMV is an important consideration in the treatment of patients suffering from Acquired Immunodeficiency Syndrome (AIDS). The defining complication is retinitis, which, if left untreated, can lead to blindness. Historically, CMV disease has been one of the more devastating of the opportunistic infections (OI) that beset HIV-1-infected individuals whose CD4.sup.+ T cell level diminishes below 100/mm.sup.3. Other disease manifestations of CMV viremia also appear as the CD4.sup.+ T cell counts drops below 100/mm.sup.3, including encephalitis, enteritis and pneumonia. At autopsy there is multi-organ involvement of CMV disease in the preponderance of AIDS patients who had severe CMV retinitis.
Patients infected with HCMV often suffer impairment of some of their vital organs, including the salivary glands, brain, kidney, liver and lungs, as a result of the effects of the disease. Furthermore, HCMV is associated with a wide spectrum of classical syndromes including mononucleosis and interstitial pneumonia. HCMV also has an oncogenic potential and a possible association with certain types of malignancies including Kaposi's sarcoma.
HCMV can cause opportunistic infections resulting in a variety of complications in, for example, immunosuppressed organ transplant patients. Prior to the use of antiviral chemotherapy, HCMV infection had been responsible for a substantial proportion of post-bone marrow transplantation (BMT) complications (J. Meyers et al., J. Infect Dis. 153:478-488 (1986)). The advent of drugs such as ganciclovir with substantial anti-CMV activity dramatically reduced complications associated with post-BMT CMV infections (G. Schmidt et al. New England J. Med. 324:1005-1011 (1991) and J. M. Goodrich et al., New England J. Med. 325:1601-1607 (1991)). Ganciclovir is most effective when administered prophylactically before diagnosis of HCMV infection. This approach has several negative aspects including a higher proportion of recipients becoming neutropenic (one third) and increased numbers of concomitant fatal bacterial and fungal diseases (J. M. Goodrich et al., Ann. Intern. Med. 118:173-178 (1993)). An alternative approach in which ganciclovir was given when HCMV antigens or DNA are first detected by culture methods provided no survival advantage compared to prophylaxis or treatment post-disease for all patients (D. J. Winston et al., Ann. Intern. Med. 118:179-184 (1993)). Finally, because of the acute nature of the side-effects, there is a need for increased hospitalization and growth factor administration to treated patients which, coupled with the cost of ganciclovir prophylaxis, increases the cost of BMT after-care.
Because human cytomegalovirus is relatively common, yet is associated with extremely serious health conditions, a considerable effort has been made to study the biology of the virus with the aims of improving diagnosis of the disease as well as developing preventative and therapeutic strategies.
The mounting of a CD8.sup.+ TL response is believed to be an important mammalian host response to certain acute viral infections. The observations that HCMV infection is widespread and persistent, and may be reactivated and become clinically evident in the immunosuppressed patient, have suggested that virus-specific T-cells, including HCMV-specific CTL, play an important role in the control of persistent infection and the recovery from CMV disease.
In humans, protection from the development of CMV disease in immunosuppressed BMT recipients correlates with the recovery of measurable CD8.sup.+ CMV-specific class I MHC-restricted T cell responses (Quinnan et al., N. Eng. J. Med. 307:7-13 (1982); Reusser et al., Blood 78:1373-1380 (1991)). These observations led investigators to carry out clinical trials in which donor-derived CMV-specific CD8.sup.+ CTL were infused into BMT recipients as an alternative to ganciclovir prophylaxis and therapy (S. R. Riddell et al., Science 257:238-241 (1992)). The transfer of CD8.sup.+ CTL clones to allogeneic bone marrow transplant recipients results in detectable CTL-based CMV immunity, and statistically significant diminution of CMV disease after BMT (E. A. Walter et al., N. Eng. J. Med. 333:1038-1044 (1995)).
Although successful in application, this approach has the disadvantage that it requires a sophisticated laboratory setup (which is also highly labor-intensive and costly) to derive the HCMV-specific CTL in vitro to be reinfused into a patient. A desirable alternative would be to deliver a vaccine derived from HCMV that would impart immunity to a BMT recipient, a solid organ recipient, a heart patient, an AIDS patient or a woman of child-bearing years, without the need for ex vivo expansion of HCMV-specific CTL. To develop such a vaccine, the viral proteins which cause the host to recognize HCMV in a protective manner must be identified, so that their amino acid sequence information can be determined. No such vaccine presently is available, however.
The viral life cycle provides insight as to the most effective time frame for targeting a vaccine to maximally disrupt virus production and spread. Following HCMV entry into the host cell and uncoating, the viral genome is expressed sequentially via immediate early (0-2 hour), early (2-24 hour) and late (&gt;24 hour) viral proteins. However, certain viral structural proteins such as pp65 are chaperoned into the cell because of their existence in large quantity in the viral particle. Much attention has focused upon structural virion proteins as potential immunodominant target antigens for HCMV-specific CTL responses.
One viral structural protein, pp65, has been identified as a target antigen for CMV-specific class I MHC restricted CTL derived from the peripheral blood of most asymptomatic CMV seropositive individuals (E. Mclaughlin-Taylor et al., J. Med. Virol. 43:103-110 (1994)). Importantly, CD8.sup.+ class I MHC restricted CTL specific for pp65 will recognize autologous HCMV-infected cells without the requirement for viral gene expression, presumably as a result of processing of the internal depot of pp65 that is transferred into the cell during infection (M. J. Gilbert et al., J. Virology 67:3461-3469 (1993)). CTL against pp65 or pp150 (another matrix protein that is recognized frequently) are able to recognize and lyse HCMV-infected cells in vitro within an hour of infection in the absence of viral gene expression (S. R. Riddell and P. D. Greenberg, Curr. Top. Microbiol. Immunol. 189:9-34 (1994)). Thus, these CTL may represent an important effector cell for limiting HCMV reactivation and progression to CMV disease, and such a cellular immune response in both immunocompromised and normal individuals would be extremely important (C.-R. Li et al., Blood 83:1971-1979 (1994)). Alternatively, CTL recognizing envelope proteins are not a substitute for pp65 and pp150 CTL because they are rarely found, arising late in infection and they are poor lytic effectors because of the down-regulation of the required Class I MHC molecules (M. J. Gilbert et al., J. Virology 67:3461-3469 (1993)). Finally, the HCMV major protein IE, produced abundantly early after infection, is specifically inhibited from being a stimulator of CD8.sup.+ CTL by a CMV-dependent blockade of its presentation (M. J. Gilbert et al., Nature [London] 383:720-722, 1996). Therefore, vaccines stimulating immunity against pp65 or pp150 would be the preferred mechanism for eliciting protective immunity against CMV infection.
It has been established that individual MHC Class I molecules preferentially bind peptides of a given motif and that the amino acid sequence of specific positions of the motif are invariant, allowing a given peptide to bind to MHC Class I molecules with high affinity. These are referred to as "anchor positions" (K. Falk et al., Nature 351:290-296 (1991)). Later studies have suggested that amino acid positions other than the anchor positions also contribute to the specificity of peptide binding to MHC Class I molecules. Additionally, residues at positions within the CTL epitope which do not interact with MHC may interact with T cells, presumably by binding the T Cell receptor (TCR). The binding of peptide amino acid residues to MHC or TCR structures is independently governed, so that substitution of TCR binding amino acid residues in many cases will not interfere with binding to the MHC molecule on the surface of an antigen presenting cell.
Edman degradation followed by N-terminal sequence analysis has been used to sequence the peptide mixture which is bound to the MHC class I peptide binding groove. In most cases the length of these peptides is between 9 and 11 amino acids. Mass spectrometry of HPLC separated peptide mixtures can elucidate the primary sequence of individual peptides. Peptide fragments which bind to MHC identified in this manner are referred to as "naturally processed epitopes." Alternatively, one can predict which peptides of a given length, between 9-11 amino acids, will optimally bind to individual HLA Class I alleles based on their conformity to a motif (K. Falk et al., Nature 351:290-296 (1991)). One such motif has been established for HLA A*0201. Positions 2 and 9 of a nonapeptide are anchor residues for HLA A*0201, with minor contributions to binding from positions 1, 4, 3, 5, 6, 7, 8 in decreasing order of importance to binding strength (J. W. Drijfhout et al., Human Immunology 43:1-12 (1995)). Similar motifs have been established for decamers and undecamers for HLA A*0201. Correspondingly, unique amino acid motifs have been established for a subset of other HLA A and B alleles to predict binding peptides between 8-11 amino acids (H. G. Rammensee et al., Immunogenetics 41 (4):178-228 (1995)).
It is recognized that CTL are an important mechanism by which a mammalian organism defends itself against infection by viruses and possibly cancer. A processed form of, e.g., a viral protein minimal cytotoxic epitope (MCE) in combination with MHC Class I molecules is recognized by T cells, such as CD8.sup.+ CTL. Functional studies of viral and tumor-specific T cells have confirmed that an MCE of 8-12 amino acids can prime an antigen presenting cell (APC) to be lysed by CD8.sup.+ CTL, as long as the APC expresses on the cell surface the correct MHC molecule that will bind the peptide.
It has been shown that the route of entry of a protein into the cell determines whether it will be processed as an antigen bound to either MHC Class I or II molecules. The endogenous or Class I pathway of protein degradation is often used by infectious viruses when they are present within cells. Viral nucleoproteins which may never reach the cell surface as full length molecules are still processed within the cell, and degraded portions are transported to the surface via MHC Class I molecules. Viral envelope glycoproteins, merely because they are cell surface molecules, do not obligatorily induce CTL recognition. Rather, it has been found that viral nucleoproteins, predominantly in the form of processed epitopes are the target antigens recognized by CD8.sup.+ CTL (A. Townsend et al., Philos. Trans. R. Soc. Lond.(Biol). 323:527-533 (1989)).
As it has become apparent that antigens entering the cell through exogenous pathways (pinocytosis, etc.) are not typically processed and presented by Class I MHC molecules, methods to introduce proteins directly into the cytoplasm have become the focus of vaccine developers. An approach that had gained favor was to use recombinant vaccinia viruses to infect cells, delivering a large amount of intracellular antigen. The enthusiasm for using vaccinia viruses as vaccines has diminished, however, because these viruses have the potential to cause disease in immunosuppressed people, such as BMT recipients. Another approach to vaccination is to mix an antigenic protein with an adjuvant and introduce the mixture under the skin by subcutaneous injection.
Yet another potential approach to immunization to elicit CTL is to use the MCE defined for a viral antigen in the context of a particular MHC restriction element to boost a CTL memory response to a virus. The ability of an MCE to provide protective immunity to challenge by a lethal dose of an infectious virus has been discussed in the literature. Vaccine developers have developed increasing interest in utilizing the MCE as the vaccine because it is capable of binding to MHC Class I molecules through external binding of the cell surface molecules without the need for internalization or processing. The MCE has been most effective as an immunogen when synthesized as a lipidated peptide together with a helper CD4 epitope (A. Vitiello et al., J. Clin. Invest. 95:341-349 (1995) and B. Livingston et al., J. Immunol. 159:1383-1392, 1997). Other modifications of the bivalent vaccine include inclusion of a signal sequence (KDEL) for endoplasmic reticulum retention and targeting to attain maximum activity. There is also evidence in the literature that an MCE presented by particular types of APC (e.g. dendritic cells) may cause a primary immune response to occur in the absence of viral infection or prior contact with the virus or tumor cell.
Accordingly, in spite of significant efforts towards identifying the HCMV proteins that are recognized by CTLs, as well as the specific identification of the HCMV late structural protein pp65, improved methods of preventing and treating HCMV infection are needed. Introduction of CMV-specific CTL into a recipient is not a universally applicable and practical strategy to confer immunity to all those at-risk individuals who may need to be immunized against HCMV infection.